Sources and Types of Gravitational Waves

Any object with mass that accelerates (which in science means changes position at a variable rate, and includes spinning and orbiting objects) produces gravitational waves, including humans and cars and airplanes etc. But the gravitational waves made by us here on Earth are much too small to detect. In fact, it isn’t even remotely possible to build a machine that can spin an object fast enough to produce a detectible gravitational wave – even the world’s strongest materials would fly apart at the rotation speeds such a machine would require.

Since we can’t generate detectable gravitational waves on Earth, the only way to study them is to look to the places in the Universe where they are generated by nature. The Universe is filled with incredibly massive objects that undergo rapid accelerations (things like black holes, neutron stars, and stars at the ends of their lives). In order to understand the types of gravitational waves these objects may produce, LIGO scientists have defined four categories of gravitational waves, each with a unique “fingerprint” or characteristic vibrational signature that the interferometers can sense and that researchers look for in LIGO’s data. These categories are: Continuous Gravitational Waves, Compact Binary Inspiral Gravitational Waves, Stochastic Gravitational Waves, and Burst Gravitational Waves. Each of these kinds of gravitational-wave generators is described below.

Continuous gravitational waves are produced by a single spinning massive object, like an extremely dense star called a neutron star. Any bumps or imperfections in the spherical shape of this star will generate gravitational waves as the star spins. If the spin rate of the star stays constant, so too do the properties of the gravitational waves it emits. That is, the gravitational wave is continuously the same frequency and amplitud (like a singer holding a single note), hence, “Continuous Gravitational Wave”. Researchers have created simulations of what they think an arriving continuous gravitational wave would sound like if the signal LIGO would detect was converted into a sound (based on the frequency of LIGO's laser's flicker). Click on "Continuous Gravitational Wave Signal" below to hear what the gravitational waves from a spinning neutron star would "sound" like to LIGO.

The next class of gravitational waves is Compact Binary Inspiral. Compact binary inspiral gravitational waves are produced by orbiting pairs of massive and dense (hence "compact") objects like white dwarf stars, black holes, and neutron stars. There are three kinds of "compact binary" systems in this category of gravitational wave generators:

Binary Neutron Star (neutron star-neutron star) or BNS

Binary Black Hole (black hole-black hole) or BBH

Neutron Star-Black Hole Binary (NSBH)

Each binary pair creates a characteristic series of gravitational waves, but the mechanism of wave-generation is the same across all three; it's called, "inspiral".

Inspiral occurs over millennia as pairs of these dense compact objects revolve around each other. As they orbit, they send off gravitational waves, which remove some of the system's orbital energy (energy which keeps them from colliding). Over eons, as the objects revolve and lose this energy, they inch closer and closer together. Unfortunately, moving closer causes them to orbit each other faster, which causes them to emit more and stronger gravitational waves, which causes them to lose more orbital energy, inch ever closer, orbit faster, lose more energy, move closer, orbit faster... etc. The stars are now doomed, inescapably locked in a runaway accelerating spiraling embrace.

This computer simulation shows the collision of two black holes, as observed for the first time ever by the LIGO on September 14, 2015. The black holes in the animation are based on the actual data from the collision as detected by LIGO. (Simulating eXtreme Spacetimes (SXS) Project (http://www.black-holes.org))

This process is analagous to a spinning figure skater. Imagine that the skater's outstretched fists are neutron stars or black holes, and the skater's body is the force of gravity binding them together. As the spinning skater pulls their fists in toward their body (i.e. as the objects orbit closer and closer), they spin faster and faster.

Unlike the skater, however, the pairs of stars or black holes cannot halt their rotation. The process of emitting gravitational waves and orbiting closer and closer sets off an unstoppable sequence of events at the end of which the two objects will collide, causing one of the Universe's most cataclysmic events. This is why the process is called “Inspiral”. Click on the image at right for an illustration of the inspiral sequence of two neutron stars.

Compact Binary Inspiral Gravitational waves are characteristically short in duration (several seconds to less than a second long) and increase in frequency as the stars orbit ever-faster. The expected gravitational wave signals from merger of neutron stars and black holes have been modeled into audible signals based on the frequencies of the gravitational waves as they would arrive at LIGO's detectors. They're called “chirps”. Click on the images below to compare the chirps from merging black holes and merging neutron stars.

Binary Black Hole "chirp" (SXS Collaboration)

Binary Neutron Star "chirp" (Image: NASA/Goddard Space Flight Center)

Lucky for LIGO, astronomers no longer have to wait to hear what real gravitational waves 'sound' like. On September 14, 2015 LIGO made scientific history by detecting gravitational waves for the very first time. The signal received by LIGO was converted into an audible sound just like the above examples. Click on the video below to hear the sound of the very first gravitational waves ever detected.

This first detection is a spectacular discovery: the gravitational waves were produced during the final fraction of a second of the merger of two black holes with masses about 29 and 36 times that of the Sun into one single, more massive spinning black hole 1.3 billion light years away! This kind of collision had been predicted but never observed.

While LIGO's very first detected gravitational waves were generated by massive black holes, it still must strain to 'hear' anything over the ever-present noise generated by everything from internal fluctuations of the laser beam itself, to traffic on nearby roads, to earthquakes happening all over the world. For a more realistic sense of what LIGO is contending with in its search for gravitational waves, click on the link below, which illustrates what a Neutron Star Merger chirp would sound like buried in other vibrational noise that LIGO can detect. You'll have to listen very carefully to hear the chirp:

Detecting gravitational waves from the Big Bang will allow us to see farther back into the history of the Universe than ever before. (Image: R. Williams (STScI), the Hubble Deep Field Team, NASA)

Astronomers predict that there are so few significant sources of continuous or binary inspiral gravitational waves in the Universe that LIGO doesn't worry about the possibility of more than one passing by Earth at the same time (producing confusing signals in the detectors). However, we do know that many small gravitational waves are passing by from all over the Universe all the time, and that they are mixed together at random. These small waves from every direction make up what we call a “Stochastic Signal”, so called because the word, 'stochastic' means, having a random pattern that may be analyzed statistically but may not be predicted precisely. These will be the smallest (i.e. quietest) and most difficult gravitational waves to detect, but it is possible that at least part of this stochastic signal may originate from the Big Bang. Detecting the relic gravitational waves from the Big Bang will allow us to see farther back into the history of the Universe than ever before. Click on the image at left to hear a simulated "Stochastic Signal" (SXS Collaboration, http://www.black-holes.org), which just sounds like a lot of random noise or static.

Burst Gravitational Waves

The search for 'burst gravitational waves' is truly a search for the unexpected—both because we’ve never detected them directly before, and because there are still so many unknowns that we really don’t know what to expect or what we might find. Sometimes we don’t know enough about the physics of a system to predict how gravitational waves from that source will appear. We also expect to find gravitational waves from systems we never knew about before. To search for these kinds of gravitational waves, we cannot assume that they will have well defined properties like the continuous and compact binary inspiral signals do. This means we cannot restrict our analyses to searching only for the signatures of gravitational waves that scientists have predicted. Searching for burst gravitational waves is an exercise in being utterly open-minded. For these kinds of gravitational waves, scientists must maintain an ability to recognize when a noticeable pattern of signals arrives, even when such a signal has not been predicted or modeled (what we think a signal may look like) before. If you don’t know what you’re looking for, it’s really hard to find it! While this makes searching for burst gravitational waves difficult, detecting them has the greatest potential to reveal revolutionary information about the Universe that we may never have learned any other way.